The fabrication of integrated circuit devices typically involves a number of photolithography steps. With lithography, a mask is used to imprint positive or negative pattern images onto a layer of photoresist coating the surface of a semiconductor wafer. The patterns thus applied define the various regions of the integrated circuit device, such as contact windows, gate electrode areas, bonding pad areas, implantation regions, and so forth. Once the resist patterns are imprinted on the device, selective processing techniques are applied to form a device structure or layer. Typically, fabrication of an integrated circuit requires a series of separate masks and lithographic transfer steps. In this way, the patterns from each mask are transferred, layer by layer, onto the surface of the wafer. Masks used in fabricating small feature sizes typically have comprised glass plates covered with hard-surface materials such as chromium, iron oxide, or molybdenum silicide.
Various lithographic techniques are known. Traditionally, integrated circuit exposure tools have involved optical systems using ultraviolet light, but as lithographic technology has advanced, other forms of lithography have evolved such as electron beam, x-ray, and ion beam lithographies. Early forms of lithography involved contact or proximity printing, in which the mask and wafer are placed in direct contact or in close alignment with one another. Due to mask damage associated with those techniques, projection printing has emerged as the preferred technology. See, e.g., U.S. Pat. No. 5,602,619, "Scanner for Step and Scan Lithography System," issued Feb. 11, 1997 to Sogard, assigned to Nikon Precision, Inc. (the "Nikon patent"), hereby incorporated by reference.
Conventional direct-write electron-beam lithography offers advantages, for example, good resolution and large depth of focus, but it has traditionally suffered in terms of its relatively low throughput levels. Projection electron-beam lithography systems have typically used full-field illumination of the mask and an absorption-transparency mask structure to take advantage of good resolution and depth of focus while attempting to capitalize on the higher throughputs achievable with projection systems. E-beam absorption-transparency masks use an aperture approach: certain electrons are blocked by the mask from hitting the wafer surface, while others are allowed to pass through apertures in the mask to contact the wafer. In these systems, increasing the beam current sufficiently to achieve commercially-viable throughput levels results in two principle difficulties: (1) stochastic electron-electron interactions produce an uncorrectable image blur at the wafer surface leading to reduced process latitude which ultimately can limit throughput; and (2) the absorbing mask structure is subject to thermal distortion. These effects vary with voltage, that is, the image blur is reduced at higher voltages but thermal distortion increases with increased voltage. For further background regarding electron-beam lithography, see, e.g., U.S. Pat. No. 5,424,173, "Electron Beam Lithography System and Method," issued Jun. 13, 1995 to Wakabayashi et. al, assigned to Hitachi Ltd., and U.S. Pat. No. 5,674,413, "Scattering Reticle for Electron Beam Systems," issued Nov. 12, 1995 to Pfeiffer et al., assigned to International Business Machines Corp., both of which are incorporated herein by reference.
A modern process for high resolution patterning comprises SCattering with Angular Limitation in Projection Electron-beam Lithography (known as "SCALPEL"), disclosed in U.S. Pat. No. 5,260,151, "Device Manufacture Involving Step-and-Scan Delineation," issued Nov. 9, 1993 to Berger et al., and assigned to Lucent Technologies, Inc. (the present assignee) (the "Berger '151 patent"), which is hereby incorporated by reference. The SCALPEL system employs charged-particle beam lithography, thus offering the depth of focus advantages of e-beam lithography, yet it uses membrane-based, scattering-type masks, not absorption-transparency masks, avoiding the thermal distortion encountered in previous attempts. The SCALPEL masks form a pattern while allowing the electron beam to pass through the membrane masks, rather than blocking electrons. Significant advantages are thus provided by SCALPEL as the transparency improves with increased voltage.
The SCALPEL system is simplistically illustrated in FIG. 1. As shown in FIG. 1, the system essentially comprises an electron beam gun 10 for emitting a beam of electrons 12, a membrane-based mask 40 for modulating the beam, one or more projection lenses 20, for demagnifying and focusing the projected beam, an aperture filter 22 for filtering strongly-scattered electrons, and the wafer 30 having a resist system 32 on its surface. Preferably, the resist system comprises a high-resolution, single layer resist. The mask comprises a membrane portion 42 and a scattering portion 44. Applying the SCALPEL concept, the membrane portion 42 is fabricated with a low atomic number material (e.g., silicon nitride), while the scattering portion 44 is fabricated with a high atomic number material (e.g., tungsten). In this way, electrons passing through both portions of the mask (e.g., beam A), scatter more strongly than electrons passing through only the low atomic number membrane portion (e.g., beam B). The aperture filter 22 blocks the strongly-scattered electrons (E.sub.S), while allowing the membrane-only trajectory (beam B) to pass with little change. Image formation is thus achieved by capitalizing on the different scattering properties of the materials used to fabricate the mask.
With this system, the components of the mask depart from the traditional chrome-on-glass configuration. In previous embodiments, the membrane mask was comprised of a silicon-nitride membrane 42 coated with a scattering-portion 44 of tungsten and chromium. The SCALPEL mask also has been designed having struts and skirts for enhancing the mechanical integrity and providing image precision, respectively (see Berger '151 patent, col. 5, lines 35-45). For further background regarding the SCALPEL system and related technology, reference is made to the following U.S. patents, all of which are assigned to Lucent Technologies, Inc., the present assignee, and which are incorporated herein by reference: U.S. Pat. No. 5,663,568, "Apparatus for Controlling a Charged Particle Beam and a Lithographic Process in which the Apparatus is Used," issued Sep. 2, 1997 to Waskiewicz; U.S. Pat. No. 5,316,879, "Sub-micron Device Fabrication Using Multiple Aperture Filter," issued May 31, 1994 to Berger and Liddle (an inventor herein); and U.S. Pats. Nos. 5,258,246, 5,130,213 and 5,079,112, issued November 1993, July 1992, and January 1992, respectively, all three of which are titled "Device Manufacture Involving Lithographic Processing," issuing to Berger et al. Background on SCALPEL is also described in L. R. Harriott & J. A. Liddle, "Projection Electron Beam Lithography: SCALPEL," FUTURE FAB INTERNATIONAL (Technology Publishing, Ltd, 1997), at 143-48, also incorporated by reference.
As with other lithography and charged-particle delineation systems in general, the success of the SCALPEL system is related to the types and function of the masks used. The invention provides an improved mask for use in a charged-particle beam lithography process having particular applicability to the SCALPEL system. Further advantages may appear more fully upon considering the description given below.